This invention relates to a transformer suitable for using as a power transformer for cycloconverters, and more particularly to such a transformer having a three-phase connected winding arrangement.
Japanese Laid-open Patent Application (Kokai) NO. 63-86563 (1988) discloses a circulating current type cycloconverter in which the transformer of the type described above is employed. The conventional cycloconverter will be described with reference to FIGS. 7 through 9 of the accompanying drawings. Reference numeral 1 designates a three-phase AC power source. A three-phase transformer 2 comprises power supply windings 3 (primary winding), positive group windings 4 (secondary winding) and negative group windings 5 (secondary winding). A positive group converter 6 comprises six thyristors 7 three-phase bridge connected. A negative group converter 8 also comprises six thyristors 9 three-phase bridge connected. Circulating current limiting reactors 10 and 11 are connected between positive and negative DC output terminals of the converters 6, 8 respectively. A load 12 is connected between neutral points of the reactors 10, 11. When a gate signal having a predetermined pattern is supplied to a gate of each thyristor 7, 9 of each converter 6, 8, voltages e.sub.op, e.sub.on and e.sub.o having waveforms shown by bold solid lines in FIG. 8 are obtained between output terminals of the positive group converter 6, between output terminals of the negative group converter 8, and between terminals of the load 12 respectively. In this case the voltage e.sub.o is a mean value of the voltages e.sub.op, e.sub.on. In FIG. 8, thin solid lines denote output voltage waveforms of the three-phase AC power source 1 and dotted lines denote fundamental wave components of the respective voltages e.sub.op, e.sub.on and e.sub.o. The cycloconverter is thus employed as a frequency changing circuit for directly changing a power source frequency into an optional frequency in a range lower than the power source frequency by controlling the gate signals supplied to the gates of the thyristors.
In the above-described cycloconverter, a positive half-wave current i.sub.op of a load current i.sub.o is supplied from the thyristors 7 of the positive group converter 6 and a negative half-wave current i.sub.on of the load current i.sub.o is supplied from the thyristors 9 of the negative group converter 8, as is shown in FIG. 9. In the case of the positive half-wave current i.sub.op, for example, the converter 6 performs a forward converting function in a period that the thyristors 7 are forward biased, such as in the period between t2 and t3, since the current is also forward biased. Furthermore, the converter 6 performs a reverse converting function in a period that the current flows in the forward direction, such as in the period between t3 and t4, after the thyristors 7 has transferred to a reverse biased state. The other converter 8 is on standby to thereby block the current flowing therethrough in the periods that the converter 6 performs the forward and reverse converting functions. Although such a standby mode is provided each half-cycle, a circulating current actually flows into both of the converters 6, 8 through the respective reactors 10, 11 since the voltages contain harmonics, as shown in FIG. 8. Only the half-wave current flows into the positive and negative group windings 4, 5 of the transformer 2 while a full-wave current flows into the power supply windings 3 so that the positive and negative group windings 4, 5 have one and the same ampere-turn. Accordingly, a current carrying capacity of the power supply windings 3 is approximately 2 where the current carrying capacity of the positive or negative group windings 4 or 5 is 1. The cycloconverter as shown in FIG. 7 is generally referred to as that of the six pulse bridge type.
In a three-phase transformer employed in the above-described cycloconverter as the power transformer, each core leg is provided with concentrically disposed one positive group winding 4, one negative group winding 5 and one power supply winding 3 interposed between the positive and negative group windings, in each phase of the three-phase windings. Accordingly, six converters and three three-phase transformers are required when a three-phase load is connected to the cycloconverter. The increase in the number of three-phase transformers to three results in increases in the production cost and the installation space of the cycloconverter. Furthermore, a no-load loss or in particular, an iron loss increases as the volume of the core is increased. The increase in the no-load loss is contrary to the employment of the cycloconverter for the variable speed control of the induction motor for the purpose of saving the consumption of the electrical energy.